First, divide both sides by 3. I think your mistake was that you were multiplying -3. Please note that in order to cancel something out you need to do the reverse operation. The reverse operation of subtraction is addition and the reverse of multiplication is division.
3(a - 5) = -6
a - 5 = -2
Now add 5 to both sides
a - 5 = -2
+5 +5
a = 3
Hope this helps!
